Hot melt pressure sensitive adhesives are 100 percent solid materials; and therefore, no solvent or water removal is required in coating operations. Also, they require a fraction of the energy required by solvent and water-based systems during coating operations. Hot melt adhesives are applied in a molten state to a backing and are then cooled rapidly with the help of a chill roll. Coating operations can be carried out at higher speeds with less equipment, lower energy cost, and greater safety than with solvent or water-based adhesives. Hot melt coating technology offers superior economics to solvent or water-based coating.
There are three classes of hot melt pressure sensitive adhesives:
1. formulated polyethylene copolymers
2. synthetic thermoplastic rubber elastomers formulated with tackifiers and
3. polyacrylates.
To date, the multi-component systems, especially the thermoplastic rubber with additives, have received more attention because of their availability, low cost and flexibility of formulation. On the other hand, the acrylic hot melt adhesives have deficiencies in that they exhibit an undesirable melt temperature/viscosity profile, unbalanced adhesive properties, and often require additives to modify the acrylic polymer in order to have acceptable performance.
Many hot melt adhesives presently available are blends of thermoplastic elastomers with plasticizer and tackifier resins derived from natural resins. These adhesives generally exhibit poor clarity, are discolored and often irritate the skin, which are undesirable for surgical tape products. Many of the elastomers used in these hot melt adhesives contain unsaturated chemical bonds, e.g., styrene-isoprene-styrene and styrene-butadiene block copolymers; these bonds are attacked by oxygen and ultraviolet radiation resulting in loss of adhesive properties. Prior art acrylic hot-melt pressure sensitive adhesives have been prepared by utilizing nonpermanent intermolecular cross-linking. A conventional solvent or water-based 100% solids acrylic pressure sensitive adhesive is far from being sufficiently fluid at the coating temperatures required for hot melt adhesives.
Hot melt adhesives based on the principle of crystallization to control adhesive viscosity almost always require long sequences of methylene groups which act to increase the effective molecular weight at low temperature. However, a balanced ratio between the crystalline domains and amorphous regions is essential. At higher crystalline content, an adhesive tends to lose its tackiness due to phase separation whereas at a lower crystalline content an adhesive of low cohesive strength is obtained.
In the past, attempts have been made to produce acrylic-based hot melt adhesives by incorporating 0.5 to 25% by weight of metallic chelating agents, such as zinc or cadmium salts to an amine-containing copolymer. The thus produced adhesives possess non-permanent, reversible bonds and showed an increase in Williams Plasticity number from 0.93 to 1.25 mm (U.S. Pat. No. 3,925,282).
Another method found in the patent literature for improving cohesive strength of acrylic-based hot melt adhesives is blending two copolymers, one copolymer containing a tertiary amine group, and the other has a built-in organic acid group. Upon mixing, a reversible ionic bonding is formed. (U.S. Pat. No. 4.045,517).
An improved hot melt pressure sensitive adhesive requires higher Williams Plasticity number to avoid cold flow. An adhesive for medical usage generally has higher requirements, such as no skin irritation, clarity, colorless, and higher moduli for lower adhesive transfer to skin. Hot melt adhesives made from amine-containing monomers have high tendency of discoloring, and metallic chelating agents may cause skin irritation. Currently available hot melt pressure sensitive adhesives exhibit acceptable tack, however, the cold flow, creep resistance and low modulus properties need improvement.
The novel hot melt pressure sensitive adhesives of this invention utilize a novel acrylic monomer, 1(2)-methyl-3-oxa-4-oxo-5-aza-nonyl methacrylate, and contain neither organic amine nor acid functional groups, nor metallic salts. The hot melt adhesives of this invention are comprised of a combination of two acrylic-based copolymers, one of which copolymers is a low glass transition temperature (Tg) copolymer, while the other is a high Tg copolymer. Each of these copolymers uses as a comonomer the novel compound 1(2)-methyl-3-oxa-4-oxo-5-aza-nonyl methacrylate which exists in two isomeric forms, as depicted below: ##STR1## For purposes of the present invention, either isomeric form, or a mixture of both, can be used as one of the co-monomers.
The pressure sensitive adhesives of the present invention differ from the prior art. Hot melt pressure sensitive adhesive compositions are disclosed in U.S. Pat. Nos. 4,045,517; 3,925,282; and 4,164,614. The closest known art is U.S. Pat. No. 4,045,517 (1977) of Guerin, Hutton, Miller and Zdanowski assigned to Rohm and Haas Company. This patent discloses novel polyacrylic hot melt adhesives prepared by blending a polymer having a Tg of -85.degree. to 10.degree. C. with a polymer having a Tg of 20.degree. to 150.degree. C. The hot melt adhesives contain carboxylic acid, sulfonic acid or amine groups.
U.S. Pat. No. 3,925,282 (1975) (Davis, Skoultchi and Fries), National Starch and Chemical Corporation, discloses a hot melt adhesive composition which possesses the reversible properties of strong cohesive strength at ambient temperatures and desirable melt viscosity at application temperatures and which is prepared by the reaction of a normally tacky acrylic-based random copolymer containing a tertiary amine-containing monomer with an organic metallic salt comprising a transition metal, tin or lead as the metallic component and an organic acid anion. The random copolymers are capable of forming reversible coordinate cross-links on the addition of small amounts of selected organic metallic salts.
U.S. Pat. No. 4,164,614 (1979) (W. A. Ames), Eastman Kodak Company, discloses hot melt adhesive compositions comprising terpolymers containing 2-ethylhexyl acrylate, N-vinyl-2-pyrrolidone and styrene. The terpolymers can be applied to tapes or labels.
In addition, U.S. Pat. No. 4,337,325 (1982) (K. Shah), Kendall Company, discloses pressure-sensitive adhesive compositions comprising a blend of copolymers consisting of acrylic monomers and vinyl lactam. The copolymer adhesive blend exhibits a viscosity less than 100,000 cps at 350.degree. F.